The present invention relates to an impact modifier for a vinyl chloride resin.
Conventionally, there have been various suggestions for improving the impact resistance of thermoplastic resins. For example, in the case of vinyl chloride resins it is known that a copolymer of diene-based rubber or acrylate-based rubber may be mixed thereto (Japanese Examined Patent Publication No. 39-19035/1964). Further, there are the suggestions of a method of increasing a particle size of a rubber component (Japanese Examined Patent Publication No. 42-22541/1967) and a method of lowering Tg of a rubber component (Japanese Unexamined Patent Publication Nos. 2-1763/1990 and 8-100095/1996) to improve impact resistance. However, with these methods, there are the problems that it is difficult to significantly improve impact resistance, and cost for raw material drastically increases.
Stress concentration of a molded article and formation and expansion of voids in a rubber play an important role in improving impact resistance of thermoplastic resins such as vinyl chloride resins. For obtaining stress concentration, it is essential to introduce a rubber component having elastic modulus significantly lower than that of a thermoplastic resin. Actually, optimization of the size and a shape of the rubber component has been investigated by introducing various rubbers. Formation and expansion of voids in the rubber are expected to contribute intensely to growth of shearing yield particularly having large energy absorption in an impact test, and to lead to improvement in impact resistance of a thermoplastic resin which contains a rubber.
Therefore, the method of promoting formation and expansion of voids in a rubber component at impact (under stress) of a molded article is extremely important, and formation and expansion of voids in the rubber component are considered to be significantly influenced by the cross-linking state of the rubber. Also, when a rubber component is previously made hollow, there is possibility that voids are easily propagated under stress.
Then, the relationship between void (hollow) state of a rubber component in a latex, hollow state of a thermoplastic resin molded article mixed with an impact modifier containing a rubber component, and impact strength of the molded article has been investigated by changing the amount of a cross-linking agent controlling the cross-linking condition of the rubber component. As a result, it has been found that an impact modifier comprising 30 to 90% by weight of a hollow rubber of which Tg is at most 0xc2x0 C. and an amount of a cross linking agent is 0 to 5% by weight, and which is not hollow after molding and has a void ratio of 3 to 90% by volume in a latex, and 10 to 70% by weight of a monomer or monomers comprising 60 to 100% by weight of at least one vinyl monomer selected from the group consisting of a (meth)acrylate compound, an aromatic vinyl compound and a vinyl cyanide compound, and 0 to 40% by weight of a monomer copolymerizable therewith, has an extremely large effect on improving impact resistance of a thermoplastic resin. Namely, though it is basic to provide stress concentration by introducing a rubber component (Tg is at most 0xc2x0 C.) having elastic modulus significantly lower than that of a thermoplastic resin such as a vinyl chloride resin which becomes a continuous phase of a molded article, it is also important to change the rubber particle to a void at impact (under stress) by decreasing the amount of a cross linking agent for this soft rubber component.
However, it has also been found that there is a tendency that a rubber particle collapses at molding and is finely dispersed and the rubber particle does not easily act as a stress concentration point when any cross linking agents become 0% by weight. Further it has been recognized from TEM observation and specific gravity measurement that a hollow rubber having a low content of a cross linking agent has voids filled with water in a latex, but in the processes of coagulation, thermal treatment, drying, blending and molding after graft polymerization, water in the rubber particle is removed, and after drying and molding, voids in the rubber collapse and disappear to become non-hollow.
On the other hand, it has been found that voids in the rubber particle are easily formed and expanded at impact (under stress) in an impact modifier containing a hollow rubber having voids in the rubber in a latex to significantly improve in impact strength. A larger amount of the cross linking agent used in a rubber is more suitable to keep the rubber particle in a hollow state. But judging from easiness of total expansion of voids, the largest improving effect is obtained by using an impact modifier containing a rubber which is hollow in a latex and is not hollow at drying and after molding by controlling an amount of a cross linking agent in a rubber to a small amount thereof.
Namely, the present invention relates to an impact modifier for a vinyl chloride resin, which is prepared by polymerizing 10 to 70% by weight of a monomer containing 60 to 100% by weight of at least one vinyl monomer selected from the group consisting of a (meth)acrylate compound, an aromatic vinyl compound, a vinyl cyanide compound and vinyl chloride, and 0 to 40% by weight of the other monomer copolymerizable therewith, in the presence of 30 to 90% by weight of a hollow rubber particle of which glass transition temperature Tg is at most 0xc2x0 C., a void ratio is 3 to 90% by volume in a latex state and which contains a cross linking agent in an amount of 0.1 to 5 parts by weight based on 100 parts by weight of the total rubber component monomer.
In the impact modifier, the amount of the cross linking agent is preferably 0.1 to 1.5 parts by weight.
In the impact modifier, the amount of the cross linking agent is more preferably 0.2 to 0.7 parts by weight.
In the impact modifier, an average particle diameter of the hollow rubber in a latex state is preferably 50 to 300 nm.
Further, the present invention relates to a vinyl chloride resin composition comprising a vinyl chloride resin containing at least 50% by weight of vinyl chloride, and the above-mentioned impact modifier.
There are various methods for preparing hollow rubber particles. For example, the following methods are well known (xe2x80x9cGosei Latex no Oyo (application of synthetic latex)xe2x80x9d, Takaaki Sugimura et al., p 285). The rubber particle of the present invention having voids in a latex may be prepared by any of these methods.
(a) a method wherein a W/O/W emulsion is prepared, and a monomer in the O layer is polymerized (O: lipophilic, W: hydrophilic)
(b) a method wherein core-shell particles having swellable cores are swelled at a temperature of at least the glass transition temperature of the shell layer to make hollow
(c) a method using a two-step polymerization of polymers having different solubility parameter values
(d) a method wherein a polymerizable monomer containing a cross linkable monomer and a hydrophilic monomer, and an oil substance are finely dispersed in water to make an O/W emulsion, and the monomer is polymerized to remove the oily substance
(e) a method using migration of a carboxylic acid copolymerized into a particle under acidic or alkaline condition in the particle.
There is no particular limitation for the hollow rubber component used in the present invention, as long as it is a rubber elastomer of which the glass transition temperature is at most 0xc2x0 C. But the lower the glass transition temperature is, the better. The glass transition temperature is required to be at most 0xc2x0 C., and preferably xe2x88x9220 to xe2x88x92130xc2x0 C. When the glass transition temperature is more than 0xc2x0 C., an improving effect for impact resistance remarkably decreases.
Examples of the rubber satisfying these conditions are a diene rubber, an acrylic rubber, a silicon rubber and an olefin rubber, but the rubber is not limited thereto. Examples of the diene rubber are a butadiene rubber, a styrene-butadiene rubber, an acrylonitrile-butadiene rubber and the like. Examples of the acrylic rubber are a butyl acrylate rubber, a butadiene-butyl acrylate rubber, a 2-ethylhexyl acrylate-butyl acrylate rubber and the like. Examples of the silicon rubber are a polydimethyl siloxane rubber and the like. Examples of the olefin rubber are an ethylene-propylene rubber, an ethylene-propylene-diene rubber and the like.
The cross-linking agent used for rubber polymerization prevents the rubber particle from collapsing into smaller pieces at molding. When an amount thereof is too high, the collapsing at molding does not rise at all, but voids are not easily expanded at impact (under stress) and the impact strength of the final molded article is small. Examples of the cross-linkable monomer as the cross-linking agent are allyl methacrylate, divinylbenzene, diallyl phthalate, polyethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, ethylene glycol dimethacrylate and the like. These may be used solely or in a combination use of two or more thereof. An amount of the cross-linking agent is 0.1 to 5 parts by weight, preferably 0.1 to 1.5 parts by weight, more preferably 0.2 to 0.7 part by weight based on 100 parts by weight of the rubber component monomer. When the amount thereof is more than 5 parts by weight, voids can not be easily expanded and then an improving effect for impact resistance of the obtained molded article becomes small. On the other hand, when the amount is less than 0.1 part by weight, the rubber particles collapse into pieces to decrease the improving effect for impact resistance of the resulted molded article.
When a monomer is polymerized in the presence of a hollow rubber particle, the monomer may be polymerized as it is with the hollow rubber particle to obtain a graft polymer, or a larger graft polymer may be obtained by enhancement methods such as acid enhancement and salt enhancement of a rubber particle.
Voids of a rubber which are hollow in a latex state can be confirmed by TEM observation after embedding a rubber latex in an epoxy resin and the like, dying it with ruthenium tetraoxide or the like. Further, the void ratio can be calculated by exactly measuring the particle size of a rubber latex by Microtrac UPA (Ultrafine Particle Analyzer) and the like, and then measuring light scattering strength of the same rubber latex. The void ratio of hollow rubber particle in a latex is 3 to 90% by volume, preferably 10 to 60% by volume from the viewpoint of improvement in impact resistance of the final molded article. When the void ratio is more than 90% by volume, the rubber particle may collapse at molding, and impact strength can not be stably improved. When the void ratio is less than 3% by volume, formation and expansion of voids in a rubber are not easily propagated at impact in the final molded article, and an improving effect for impact resistance decreases.
In order to maximize the improving effect, the particle size of the graft copolymer of the present invention is preferably 0.05 to 2.0 xcexcm, but the suitable value is slightly varied depending on the kind of a thermoplastic resin. Out of this range, the improving effect for impact resistance tends to decrease.
There is no particular limitation for the method for preparing hollow rubber, and the rubber can be efficiently prepared by using emulsion polymerization.
The graft copolymer of the present invention is obtained by polymerizing 10 to 70% by weight, preferably 12 to 40% by weight of a monomer in the presence of 30 to 90% by weight, preferably 60 to 88% by weight of a hollow rubber component. When the amount of the hollow rubber component is less than 30% by weight, the improving effect for impact resistance is small. When it is more than 90% by weight, a particle of the impact modifier collapses at molding the mixed composition, and the improving effect for impact resistance of the molded article becomes small.
The monomer polymerized in the presence of a hollow rubber particle is a monomer or monomers containing at least one selected from the group consisting of a (meth)acrylate compound, an aromatic vinyl compound, a vinyl cyanide compound and vinyl chloride in an amount of at least 60% by weight and the other monomer copolymerizable therewith in an amount of at most 40% by weight.
Examples of the (meth)acrylate are methacrylates such as methyl methacrylate, ethyl methacrylate, butyl methacrylate and 2-ethylhexyl methacrylate; acrylates such as methyl acrylate, ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate; and the like. Examples of the aromatic vinyl compound are styrene, xcex1-methylstyrene, chlorstyrene and the like. Examples of the vinyl cyanide compound are acrylonitrile, methacrylonitrile and the like.
Examples of the other copolymerizable monomer are (meth)acrylates other than the above-described esters such as glycidyl (meth)acrylate, and maleimide compounds such as maleimide and N-phenylmaleimide.
Examples of the vinyl chloride resin used in the present invention are a poly(vinyl chloride), and a copolymer of at least 50% by weight of vinyl chloride and a monomer copolymerizable with vinyl chloride such as vinyl acetate and ethylene, a chlorinated vinyl chloride resin and the like.
The amount of the impact modifier used is usually 1 to 30 parts by weight based on 100 parts by weight of the vinyl chloride resin. When the amount is less than 1 part by weight, an improving effect for impact resistance is not recognized. When the amount is more than 30 parts by weight, there is a tendency that a tensile strength of the obtained molded article decreases.